Low-volt discharge lamps are typically operated with the aid of an electronic ballast (EVG). The alternating current required for operating the lamp is generally generated in the ballast by means of known half-bridge inverters. The half-bridge is used to operate a load circuit that comprises one or more lamps. The load circuit comprises inductive and capacitive elements, the result being a prescribed load circuit resonant frequency.
Depending on the circuit concept, in the case of self-oscillating resonant circuits operation at the resonant frequency is set up during idling, that is to say in the starting phase of the lamp in the case of an EVG. In this case, the resonant current is determined solely by the Q factor of the resonant circuit. This leads to a very high component loading in the case of a high Q factor, since very high currents occur.
The starting voltage for starting a lamp by means of a resonant circuit, and the reactive current, associated therewith, before the starting can be limited only by the saturation behavior of the resonance inductor or by reduction of the Q factor of the resonant circuit. Consequently, to date, the no-load voltage has been limited by a resonant inductor whose saturation has deliberately been selected to be low. This measure leads to an additional increase in the resonant current. The limitation of the current is performed by a Q factor of the resonant circuit that is deliberately worsened in some circumstances. This worsening takes place, however, to the detriment of the efficiency and is practicable only for equipment of relatively low power.
A further-developed current limitation is disclosed in European Patent EP 0 798 952 B1. In the EVG described there, the control path of a transistor is arranged in emitter line of one of the inverter transistors. The effective emitter resistance of the inverter transistor is varied continuously as a function of the voltage drop across one of the resonant circuit components via the variable conductivity of this control section, and the clock frequency of the inverter is thereby increased so far that a reduction in the no-load voltage in the resonant circuit is achieved in conjunction with current limitation because of the now stronger detuning with respect to the resonant frequency of the resonant circuit.
A similar current limiting circuit is disclosed in European Patent Application EP 0 800 335 A2. An auxiliary transistor is connected in each case in the control loops of the half-bridge inverter transistors such that the emitter resistance of each half-bridge inverter transistor is formed by a parallel circuit that comprises at least one ohmic resistance and the control path, arranged parallel thereto, of the corresponding auxiliary transistor. It is thereby possible for the effective emitter resistance or the feedback of the half-bridge inverter to be switched over as a function of the operating phases of the lamp, and so for the clock frequency of the half-bridge inverter to be varied in a simple way within wide limits by the dimensioning of the resistances of the parallel circuit according to the invention. Here, as also in the previous case, the auxiliary transistor is controlled by the lamp voltage, which in turn controls the emitter line of a half-bridge transistor.